The early events of infection with Theileria parva in cattle: infectivity for cattle of leukocytes incubated in vitro with sporozoites.

Leukocytes were isolated from bovine blood and, after short periods of incubation in vitro with sporozoites of Theileria parva, were washed thoroughly, and their infectivity tested in autologous and allogeneic hosts. Using a standard inoculum of 10(6) viable cells, it was found that, after incubation in vitro for either 1 or 24 h, the cells initiated lethal infections in autologous cattle, but failed to infect allogeneic animals. Autologous and allogeneic erythrocytes and mouse lymphocytes similarly incubated with sporozoites failed to infect cattle. The supernatant from bovine lymphocyte suspensions incubated with sporozoites for 1 h produced lethal infections whereas after 24 h of incubation the supernatant was non-infective. All cattle which did not develop detectable infection were fully susceptible to subsequent challenge with a stabilate of sporozoites. By inoculating cattle with graded doses of autologous blood leukocytes which had been incubated for 24 h with sporozoites, it was found that as few as 2 X 10(3) cells gave rise to infection. The results indicate that this approach can be used to evaluate different cell populations as targets for infection and transformation by sporozoites of T. parva.